This invention relates to improved apparatus and methods for detecting harmful substances, whether airborne or grounded, whether biological or chemical, which may pose an immediate or long term hazard to human life or health.
The afore-cited patents and co-pending applications, disclose apparatus and methods for collecting various contaminants (including vapors and particles, chemical or biological) from a large volume of air into a small volume of carrier liquid, so as to permit or facilitate rapid and ultra-sensitive detection of traces of hazardous or illicit substances which may be otherwise difficult to detect. The collected contaminants may be either dissolved by or suspended in the carrier liquid.
The earliest apparatus of this type was intended mainly for the absorption of vapors by the carrier liquid and was therefore referred to as liquid-absorption air sampler. With subsequent use of the same apparatus for the collection of respirable particles, the term (absorption) became inappropriate, as the collected particles may remain suspended in the carrier liquid without being dissolved therein. Such apparatus and methods will therefore be referred to herein as (HTLAAS) for High-Throughput Liquid-Assisted Air Sampling, which applies to collected air contaminants which are either dissolved or suspended in a carrier liquid.
A good measure of the performance of HTLAAS devices is the concentration factor F, which is proportional to the ratio of the concentrations in the liquid carrier and in air of the monitored contaminant, hereinafter referred to as (analyte.) The concentration factor F is defined by the equationF=εS/vL   [1],where ε is the sampler's collection efficiency, S is its air sampling rate, and vL is the volume of liquid in which the analyte is collected. The concentration factor F can thus be enhanced by increasing ε and/or S or by decreasing vL.
The need for efficient aerosol collectors has been appreciated for more than a decade both for all point bio-detection systems and for future chemical point detectors, where the term “point” applies to spots where samples are taken as opposed to remote detection. Several collectors have been developed and used by the military and first responders. However, recent incidents of bio-terrorism have revealed serious shortcomings of these collectors. When these were used in conjunction with immunoassay-based test strips, the resulting effective detection limit for anthrax bacilli was far above the known dangerous or lethal concentrations, so that inhalation of low but lethal doses of anthrax or other biological warfare [BW] aerosol agents could have been easily overlooked. Although several existing wetted wall cyclone aerosol collectors can remove a substantial fraction of particulates from a large volume of air (several cubic meters) and transfer them into a small liquid volume (a few milliliters) for analysis, their power requirements are high (400-500 watts)—driven by the need to collect particles as small as 1 micron. Cyclones and most inertial separation devices are intrinsically very inefficient at capturing small particles.
Somewhat of an exception may be the PHTLAAS [Portable HTLAAS] of U.S. Pat. No. 6,087,183, a variant of which was found to yield a collection efficiency of 66±3% for 1-micron particles and 84±4% for 4-micron particles at an air flow rate of 317 liters/minute, as reported by Kesavan, J.; Carlile, D.; Doherty, R. W.; Sutton, T.; and Hottell, A., “CHARACTERISTICS AND SAMPLING EFFICIENCY OF PHTLAAS™ AIR SAMPLER,” ECBC-TR-267, US ARMY, 2002. When a similar sampler, hereafter referred to as “recent PHTLAAS”, was operated at full power with a 12-volt battery, the measured power consumption was only 42 watts [3.5 amps at 12 volts]. The comparatively low power consumption of only 42 watts by the recent PHTLAAS is attributed to its unique flow pattern in which the direction of the air stream is partly reversed as it enters through the air intake, as disclosed in U.S. Pat. No. 6,087,183.
Although this recent PHTLAAS seems to compare favorably with other inertial-separation-type collectors, it still cannot match the much higher efficiencies that are obtainable with electrostatic precipitation [EP] technology for removing small particles from a gas [see, e.g., Altman, R.; Buckley, W.; and Ray, I.; “WET ELECTROSTATIC PRECIPITATION DEMONSTRATING PROMISE FOR FINE PARTICULATE CONTROL,” Power Engineering, January-February, 2001, or Parker, K. R., editor; “Applied Electrostatic Precipitation,” Chapman & Hall, London, 1997].
According to the latter references, wet EP [WEP] can achieve collection efficiencies of 99.9% for particles as small as 0.01 micron in size and for various gaseous species, including dioxins/furans, which could also assure capture of toxins and dry virus particles. The latter remain suspended in air long after evaporation of water from the droplets in which they were originally dispersed and may thus present a persistent not readily noticeable hazard. Therefore, an ability to collect dry virus particles should greatly enhance the effectiveness of biological agent detection systems.
Inertial separation devices, including the PHTLAAS, operate on altogether different principles than EP and consequently have different physical structures. Whereas the airflow within the PHTLAAS is highly turbulent and swirling rapidly, so as to impel particles towards the container wall by centripetal action, the flow in EP devices is substantially laminar, so as to permit high flow rates at rather low pressure drops and low power consumption.
The major reduction in power consumption that is expected from the use of EP yields not only major savings in the size, operating costs, and equipment cost of the resulting collectors, but also smaller and lighter instruments by reducing the size and weight of required batteries or else permits uninterrupted operation between battery replacements for longer time periods, thereby further increasing the utility of portable collectors.